Collect the elderflower heads. Shake them lightly to remove any bugs then place them into a big mixing bowl. Peel the rind off the lemon in thick strips then cut the lemon into quarters and add it all to the bowl.
In a big pan, combine the sugar and water. Place over a medium heat and stir occasionally until all of the crystals have dissolved. Take off the heat and stir in the citric acid. Leave to cool until lukewarm.
When the syrup has cooled, pour it into the bowl -- it should be enough to cover everything, though a few bits may bob to the top. Cover with cling film and leave at room temperature overnight.
The next day, line a colander or deep sieve with muslin and set it into a big bowl or pan. Pour the contents of the elderflower bowl into the sieve, taking care that it doesn't overflow.
Place clean glass cordial bottles ( I use two and then a plastic bottle for freezing, which is fine just washed) in an oven set to 210° F (100° C) until they've been heated through. Let the bottles cool until they're warm.
Use a jug and funnel to pour the cordial into the bottles. It will keep in the fridge for up to a month and in the freezer (in a plastic bottle) for a few months. I like it with sparkling spring water but some prefer still. You only need a very small amount of cordial.